1. Field of the Invention
The present invention relates to measure flow of fluid.
2. Description of the Background Art
Methods for measuring the flow of fluid are generally classified into two types; heating current measurement and temperature difference measurement.
The heating current measuring method includes detecting a heating current required for maintaining a heating resistor at a constant temperature level to measure flow of the fluid. On the other hand, the temperature difference measuring method includes keeping the temperature of a heating resistor at a constant level and detecting a difference in the temperature between an upstream temperature sensing resistor provided in the upstream side and a downstream temperature sensing resistor provided in the downstream side of the heating resistor to measure the flow of the fluid. Such a temperature difference measuring method is disclosed in the form of a fluid flow measuring apparatus in Japanese Patent Publication No. 5-7659(1993).
The two conventional methods described above have the following drawbacks respectively.
A fluid flow measuring apparatus implementing the heating current measuring method is unfavorable in the downsizing and may measure a small rate of the flow with much difficulty.
On the other hand, a fluid flow measuring method employing the temperature difference measuring method is favorable in the downsizing and can measure a small rate of the flow. However, when the fluid flow is increased, the temperature of the upstream temperature sensing resistor remains not further declined, and that of the downstream temperature sensing resistor may further be declined. This will lower the sensitivity and narrow the dynamic range when the flow of fluid increases. As the temperature of the temperature sensing resistor is varied by a change in the flow, the response of the apparatus is also declined.